Method of drying wax models



United States Patent METHOD OF DRYING WAX MODELS Hendrik JohannesMeerkamp Van Embden, Casparus Antonius Nieuwenhuis, and Petrus JohannesBuijsman, all of Eindhoven, Netherlands, assignors to North AmericanPhilips Company, Inc., New York, N.Y., a corporation of Delaware NoDrawing. Filed Apr. 24, 1958, Ser. No. 730,536

Claims priority, application Netherlands May 18, 1957 1 Claim. (Cl.22-196) This invention relates to methods of drying wax models coatedwith a moist layer of a suspension of fine ceramic material, devices forcarrying out such methods and products obtained by such methods.

As is well-known, in precision casting, the casting moulds are obtainedby applying by pouring a ceramic paste to models made up of wax, whichpaste hardens after some time. The wax is removed from the resultantmould by melting, Whereafter the mould is dried and burnt to form aporous stone, into which the molten metal is introduced. During casting,the stone must satisfy several conditions, viz:

(1) It must be resistant to the casting material;

(2) It must be porous for sufiicient discharge of occluded air andliberated gases;

(3) The molten metal must not penetrate the pores or possibly cracks;

(4) It must be strong enough to resist the pressures which occur duringcasting;

- (5) The inner wall of the mould requires a smooth surface so that thecasting also acquires a smooth surface.

The contradictory requirements mentioned sub (2) and (3) are usuallyfulfilled by first coating the wax model with a thin layer of a veryfine mass, behind which a material coarser but more porous is provided.This first thin layer is applied to the wax models by immersion orspraying with a suspension of fine ceramic material containing a binder,which is subsequently dried, whereafter the moulds are obtained bypouring a coarser suspension onto these wax models.

The object of the invention is to provide means of obtaining greatconstancy in the dimensions of castings.

According to the invention, the drying process takes place at atemperature maintained constant at a value such that the wax does notsoften and also with a high relative humidity of the ambient air.

Since wax has a high coeflicient of expansion, differences intemperature may result in variations of the dimensions.

However, it has been found that maintaining a constant or substantiallyconstant temperature during these treatmetns is not sufiicient, but thatespecially after the first coating with the fine suspension a constantand high relative humidity of the air is of much greater influence.

When such wax models covered with the moist layer of the fine suspensionare dried, then according to the invention, a high relative humidity ofthe ambient air of from about to is of utmost importance for obtaining agood product." In fact, with a lower relative humidity, this thin layerdries rapidly and as a result of the heat of evaporation of the watercontained in the layer, the temperature of the outer layer and of theunderjacent wax model is considerably decreased. At room temperature andwith a low relative humidity of, for example, from 40% to 50%, thecooling could be up to 10 C. As a result thereof, the wax model contracts and has smaller dimensions at the moment when the ceramiccovering layer becomes dry and solid. When, subsequently, the wholeassumes the ambient temperature, the wax again expands, with the resultthat the ceramic layer must necessarily crack and/or blister. This looselayer is deteriorated or may partly be rinsed away during the subsequenttreatment, the pouring out of the coarse suspension. Consequently, theliquid of the coarse suspension may penetrate between the covering layerand the wax, causing the smooth surface to be roughened, or duringpouring in the molten metal this may penetrate the coarse mass of themould, so that the casting then also becomes unserviceable.

It has been previously suggested to add glycerine or other substances tothe fine suspension in order to avoid cracking, but jellification of theliquid added thus becomes incomplete or is even completely prevented sothat an adhering ceramic covering layer is not obtained.

The mean cubic coefficient of expansion of most kinds of wax between 20and 30 is a=6 10- which is a factor 10 higher than that of most metals.

A 10 C. variation in temperature of the wax model thus results in avariation in volume of 0.6%.

What is claimed is:

A method of making a precision ceramic mold comprising the steps ofapplying a thin layer of a liquid suspension of a relativelyfinelycomminuted ceramic material over a wax model, drying the layer offinely-divided material on the wax model in an atmosphere having arelative humidity of about 80 to 90% and a temperature below thesoftening point of the wax, applying over the dried thin layer offinely-comminuted material a layer of relatively coarse ceramicmaterial, and melting the wax to remove the same from the mold.

OTHER REFERENCES Wride: The Moisture Gradient and Its Eifect in theDrying of Clayware, Iowa State College Journal of Science, vol. 24,October 1949, pp. 122-124.

